PROJECT SUMMARY Recent discovery of the molecular identity of pore-forming subunit of the mitochondrial Ca2+ uniporter (encoded by MCU gene) provides new possibilities for applying genetic approaches to study the mitochondrial Ca2+ (mtCa2+) influx mechanism. T hough the alteration of MCU function and mtCa2+ overload are frequently observed in non- cardiac human diseases, cardiomyocytes (ACMs) and MCU function contribute to the pathology . Recently, I reported that a post-translational modification (PTM) of MCU (tyrosine it is still not clear how mtCa2+ in adult phosphorylation) is one of the critical regulatory mechanisms for upregulating MCU function in ACMs. PTM of MCU initiates overload, -dependent ROS overproduction and activation of apoptotic signaling under alpha1-adrenoceptor (alpha1-AR) stimulation, suggesting that PTM of MCU plays an important role in the development of cardiac dysfunction under Gq-protein-coupled receptor (GqPCR) stimulation, which is one of the major causes of heart failure (HF) in vivo. In addition to the discovery of the PTM of MCU, I identified a form of transcriptional/post-transcriptional regulation of MCU, namely the existence of alternative transcript variants (?short- form? MCU, termed MCU-S) in human and mouse in addition to the original MCU (?long form? MCU, renamed MCU-L). Importantly, MCU-S is highly expressed in non-excitable cells including adult cardiac fibroblast (ACFs). Our preliminary data show that introduction of MCU-S enhances the formation of Ca2+-permeable channels at the plasma membrane (PM). In addition, the PM-localized MCU channel (PM-MCU) formed by MCU-S activates glycolysis followed by acceleration of ATP production in the cytoplasm, possibly due to the increase in local [Ca2+]c beneath the PM. These preliminary findings indicate an important role of PM-MCU channels for energy metabolism especially in non-excitable cells (such as ACFs). We also determined that the manipulation of MCU-S/MCU-L ratio mtCa2+ mtCa2+ can secondarily modulate the driving force of MCU-channel trafficking to IMM, which eventually inhibits the mtCa2+ uptake, mtCa2+ -dependent ROS overproduction and activation of apoptotic signaling under alpha1-AR stimulation. Therefore, I hypothesize that the MCU gene encodes two isoforms that form both mitochondrial and non- mitochondrial MCU channels; novel transcript variant MCU-S forms a PM-MCU channels that regulate cell?s metabolism and inhibits the MCU-channel trafficking to IMM. To test the hypothesis, I will investigate whether MCU-S forms Ca2+-permeable MCU channels at PM and is involved in the mechanism for the activation of glycolysis to accelerate ATP production in the cytoplasm in primary ACMs and ACFs (Aim.1). I will next test whether switching the main variant from MCU-L to MCU-S protects the heart from mtCa2+-overload-mediated apoptotic death, energy depletion and cardiac dysfunction under GqPCR stimulation in vivo (Aim.2). The outcome of this project is expected to lead to a novel treatment strategy for cases of HF under GqPCR stimulation that involve mtCa2+ overload, oxidative stress, energy depletion and ACM death. Moreover, elucidation of the role of MCU variants will provide us novel insights into the molecular basis of mtCa2+ handling in the heart.